1. Field of the Invention
This invention relates generally to the field of electrocardiographic (ECG) recording of very low amplitude micropotential signals known as late potentials, and more emphatically, the teachings of this invention extend Holter applications to performing a real time analysis of late potentials while simultaneously storing, within the same Holter recording medium, multiple channels of ECG data for a standard 24 hours, or more. Late potential assessment time periods are selected during which the multiple channel ECG is sampled for the presence of late potentials. A high resolution digital average of multiple channel ECG data is stored in a Holter recording medium of analog tape, digital tape, solid state memory, rotating magnetic disc, rotating optical disc, rotating magneto-optical memory, removable memory card storage using either solid state or optical technologies, or any sequential or random storage system. This storage system will store high resolution digital data for later presentation and analysis on an ECG scanning computer. The high resolution data stored is useful in the analysis of micropotentials and specifically in determining the existence of ventricular late potentials which are evidence of ventricular tachycardia.
2. Description of the Related Art
Due to size and weight restrictions, conventional Holter recorders presently available in the medical profession employ lightweight battery-powered recorders having magnetic tape or solid-state memory for storage. The conventional recorders employing magnetic tape storage, normally store ECG recordings up to 24-hours, or more, of multiple channel ECG data. The recorded ECG data can then be played back at many times the recording speed, to generate post-recording reports. The ECG data stored on these magnetic tape based Holter recorders is typically stored in an analog format from 0.1 Hz to 100 Hz, whereas, frequencies from 10 to 250 Hz are required to perform micropotential analysis of late potentials. Conventional solid-state Holter recorders have the capability to record at of 1000 samples per second or more with between 8 to 12 bits of digital resolution which is equivalent to magnetic tape based recorders of D.C. to 300 Hz with an accurate amplitude resolution of 0.4% or better without employing calibration signals. In an effort to reduce the memory requirements of an ambulatory system, solid state Holter recorders normally employ data compaction algorithms. Numerous data compaction algorithms are available, and all result in at least a partial loss of the original data. It is well known in the art of electrocardiography that conventional Holter recorders receive signal inputs that have background noise in the range of 20 to 50 microvolts. Thus, thresholding techniques must strip away the lower 50 microvolts of a signal in attempts to reduce the noise that is inherent with the input. As a result of these limitations, conventional Holter monitoring is concentrated with signals in the range of a millivolt. The ability of these solid state recorders to record micropotentials is, therefore, subject to limitations due to size, weight, power and memory capacity to record a full 24 hours of late potential data.
The analysis of micropotentials concerns the analysis of signals in the microvolt range and requires sensitivity not attainable with conventional Holter techniques. Conventional Holter recording permits substantially higher noise levels (20 to 50 microvolts) than the signal to be measured in micropotential analyzing. One of the solutions to the problem of extraneous noise has been the use of digital signal averaging techniques, wherein, averaging is accomplished over a time period of five or ten minutes, during which time it is preferable that patient activity be at a minimum to avoid artifact. The signal averaging techniques employed by ECG analysis systems known to the art, allow the analysis of signals on the order of a microvolt by relying on the basic principle that the repetitive nature of a bioelectric signal allows the signal to be averaged over a sufficiently large number of samples. During this averaging process, nonrepetitive noise will be effectively eliminated by the signal averaging process, due to the nonrepetitive nature of the noise. The larger the number of samples that are averaged, the more effective the signal averaging techniques are at eliminating noise. By digitizing the electrocardiographic signals and converting the signals into a series of corresponding points, computational methods can then be employed to average the corresponding points of the sampled electrocardiographic signals. The results yielded are necessarily related to the resolution used in digitizing. Noise or artifact that is nonrepetitive is thereby reduced or eliminated, for all practical purposes, by signal averaging a sufficiently large sample of input signals. Thus, repetitive signals can be effectively signal averaged to eliminate noise that is of a nonrepetitive nature.
Holter techniques have also been disclosed in which the resulting recording was itself analyzed for the presence of micropotentials without the use of additional equipment. Such a system is taught by U.S. Pat. No. 4,883,065 issued to Kelen, and assigned to the assignee of the present invention, wherein conventional Holter recordings were used in the analysis of micropotentials. These conventional recordings store and record up to 24-hours of three channels of ECG data. The stored ECG data can then be used to generate post recording reports. However, the fidelity of the signals recorded on these conventional systems is lacking due to the high frequency limitation in the vicinity of 50 to 100 Hz found in most Holter systems.
As can be seen by the foregoing discussion, the disclosure of the prior art lacks in the teachings relating to micropotential analysis on an 24 hour ambulatory basis. Digital systems with sampling rates of 1000 samples per second and 12 bit digital accuracy are required for sufficient fidelity to perform micropotential analysis. Systems that can sample such high rates in real time require large amounts of memory to hold 24 hours of data, typically 500 megabytes. Currently available signal averaging systems, used in micropotential analyzing, require the patient to be wired to the system while at rest for short 5-10 minute epochs. These systems will signal average the short periods in real time and display the results but require large hard disks if all the 24 hours of data is to be stored. Therefore, there remains a need within the art for a method and apparatus capable of performing a continuous, high resolution, real time analysis of ECG late potentials that can be stored on the type of storage medium used with a Holter recording device. This is provided by the teachings of the present invention wherein a recording of a digitized average of a multiple channel ECG is used to identify and store late potentials during selected time epochs and continuously over periods of 24 hours or more.